Investigation on dissimilar underwater friction stir lap welding of 6061-T6 aluminum alloy to pure copper

Friction stir welding (classical FSW) is considered to offer advantages over the traditional fusion welding techniques in terms of dissimilar welding. However, some challenges still exist in the dissimilar friction stir lap welding of the aluminum/copper (Al/Cu) metallic couple, among which the formation of the Al–Cu intermetallic compounds is the major problem. In the present research, due to the fact that the formation and growth of the intermetallic are significantly controlled by the thermal history, the underwater friction stir welding (underwater FSW) was employed for fabricating the weld, and the weld obtained by underwater FSW (underwater weld) was analyzed via comparing with the weld obtained under same parameters by classical FSW (classical weld). In order to investigate the effect of the external water on the thermal history, the K-type thermocouple was utilized to measure the weld temperature, and it is found that the water could decrease the peak temperature and shorten the thermal cycle time. The XRD results illustrate that the interface of the welds mainly consist of the Al–Cu intermetallic compounds such as CuAl₂ and Cu₉Al₄ together with some amounts of Al and Cu, and it is also found that the amount of the intermetallic in the underwater weld is obvious less than in the classical weld. The SEM images and the EDS line scan results also illustrate that the Al–Cu diffusion interlayer at the Al–Cu interface of the underwater weld was obviously thinner than that of the classical weld.     

铝/钢异种金属的超声振动强化搅拌摩擦焊接工艺

采用新型超声振动强化搅拌摩擦焊接工艺实现了6061-T6铝合金以及QP980高强钢的搭接焊, 对比分析了有无超声作用下, 接头的宏观形貌、微观组织和拉伸剪切性能, 同时研究了超声振动对焊接载荷的影响。结果表明: 焊接前对母材施加超声振动, 可以起到软化母材的作用, 促进了材料的塑性流动, 扩大了铝/钢界面区和焊核区, 使更多的钢颗粒随搅拌针旋转进入铝合金侧, 在界面区边缘形成钩状结构, 进而提高了接头的失效载荷; 超声改变了FSW接头断裂位置和断口形貌, 提高了接头力学性能, 在本实验工艺参数范围内, 接头最大的平均失效载荷为4.99 kN; 当焊接速度为90 mm/min, 下压量为0.1 mm时, 施加超声振动使接头的平均失效载荷提高了0.98 kN, 拉剪性能提升28.24%;施加超声振动后轴向力F_z、搅拌头扭矩M_t和主轴输出功率分别下降2.46%, 6.44%和4.59%。     

Effect of welding parameters on microstructure and mechanical properties of friction stir welded joints of a super high strength Al–Zn–Mg–Cu aluminum alloy

Samples made of a super high strength aluminum alloy with high Zn content were friction stir welded with rotation rates of 350–950 rpm and welding speeds of 50–150 mm/min. The effect of welding parameters on the microstructure and mechanical properties was investigated. It was observed that the grain size of the nugget zones decreased with the increasing welding speed or the decreasing tool rotation rate. Most of the strengthening precipitates in the nugget zone were dissolved back and the intragranular and grain boundary precipitates in the heat affected zone coarsened significantly. The greatest ultimate tensile strength of 484 MPa and largest elongation of 9.4 were obtained at 350 rpm−100 mm/min and 350 rpm−50 mm/min, respectively. The ultimate tensile strength and elongation deteriorated drastically when rotation rate increased from 350 to 950 rpm at a constant welding speed of 100 mm/min.     

Defects and tensile properties of 6013 aluminum alloy T-joints by friction stir welding

In this paper, 6013-T4 T-joints were successfully fabricated with different welding parameters by friction stir welding in two different combination modes of skins and stringers. The distribution features and formation mechanisms of defects in T-joints were observed and analyzed. The effect of defects and welding parameters on tensile properties of T-joints was investigated. The result shows that the T-joint without tunnel defect only can be obtained with the traverse speed of 100 mm/min in this experiment, and the welding parameters influence the features and sizes of kissing bond defects. The fracture of T-joints along the shin is attributed to the kissing bond defect and the tunnel defect is the main factor affecting the tensile properties along the stringer.     

Microstructure at friction stir lap joint interface of pure titanium and steel

A commercially pure titanium plate was lap joined to a structural steel plate via friction stir welding, and the microstructures at the lap joint interface were intensively examined by means of electron backscatter diffraction analysis and transmission electron microscopy. Swirling-like macro- and micro-intermixing zones of titanium and steel are formed along the interface, where tiny Fe-Ti intermetallic particles are dispersed and mixed with β titanium in layers. The lap joint has high shear tensile strength, which is supposed to result from the dispersion of tiny Fe-Ti intermetallic particles and the formation of β titanium at the joint interface.     

Effect of position and force tool control in friction stir welding of dissimilar aluminum-steel lap joints for automotive applications

Widespread use of aluminum alloys for the fabrication of car body parts is conditional to the use of appropriate welding methods, especially if dissimilar welding must be performed with automotive steel grades. Friction stir welding (FSW) is considered to be a reasonable solution to obtain sound aluminum-steel joints. In this context, this work studies the effects of tool position and force control in dissimilar friction stir welding of AA6061 aluminum alloy on DC05 low carbon steel in lap joint configuration, also assessing proper welding parameter settings. Naked eye and scanning electron microscopy (SEM) have been used to detect macroscopic and microscopic defects in joints, as well as to determine the type of intermixture between aluminum and steel. The joint strength of sound joints has been assessed by shear tension test. Results point out that tool force control allows for obtaining joints with better quality and strength in a wider range of process parameters. A process window has been determined for tool force conditions to have joints with adequate strength for automotive purposes.The full text can be downloaded at https://link.springer.com/content/pdf/10.1007%2Fs40436-019-00290-1.pdf     

Friction stir welding of dissimilar materials between AA6061 and AA7075 Al alloys effects of process parameters

Dissimilar AA6061 and AA7075 alloy have been friction stir welded with a variety of different process parameters. In particular, the effects of materials position and welding speed on the material flow, microstructure, microhardness distribution and tensile property of the joints were investigated. It was revealed that the material mixing is much more effective when AA6061 alloy was located on the advancing side and multiple vortexes centers formed vertically in the nugget. Three distinct zones with different extents of materials intercalations were identified and the formation mechanism of the three zones was then discussed. Grain refinement was observed in all three layers across the nugget zone with smaller grains in AA7075 Al layers. All the obtained joints fractured in the heat-affected zone on the AA6061 Al side during tensile testing, which corresponds very well to the minimum values in microhardness profiles. It was found that the tensile strength of the dissimilar joints increases with decreasing heat input. The highest joint strength was obtained when welding was conducted with highest welding speed and AA6061 Al plates were fixed on the advancing side. To facilitate the interpretation, the temperature history profiles in the HAZ and at zones close to TMAZ were also measured using thermocouple and simulated using a three-dimensional computational model.     

Ultrasound enhanced friction stir welding of aluminum and steel: Process and properties of EN AW 6061/DC04-Joints

In this work, the joining of aluminum to steel was conducted by ultrasound enhanced friction stir welding (USE-FSW). The power ultrasound was introduced into one of the metal sheets by an ultrasonic roll seam module synchronously to the FSW-process. The effect of the ultrasound on the resulting welds, their microstructure and their corrosion properties was investigated by light and scanning electron microscopy and corrosion investigations. The USE-FSW-joints showed less and smaller steel particles in the nugget zone as well as a thinner continuous intermetallic phase of FeAl₃ at the interface. The nondestructive testing method of computed laminography proved the observations made by optic microscopy due to non-porous joints for both techniques. Corrosion investigations showed only low corrosion current densities and no enhanced galvanic corrosion for the EN AW-6061/DC04-hybrid joints in sodium chloride solution.     

The potential adaptation of stationary shoulder friction stir welding technology to steel

Stationary shoulder friction stir welding is a newly developed technique currently used for joining plates of relatively soft metals at different angular planes. The process is not currently applicable to steel, hence the present study was developed to investigate the theoretical and technical viability of stationary shoulder technology in DH36 steel. Aluminium welds were produced using both conventional rotating shoulder and stationary shoulder friction stir welding techniques, and steel welds were produced using only conventional friction stir welding techniques. The effects of stationary shoulder technology on both the microstructural evolution and resultant mechanical properties of aluminium have been evaluated so that the likely effects on steel could be predicted. In the aluminium welds, the stationary shoulder technique results in a distinct transition between stirred and unstirred material, contrasting to the gradual change typically seen in conventional friction stir welds produced with a rotating shoulder. An investigation of weld properties produced in DH36 steel has demonstrated that if the stationary shoulder weld technique was used, the microstructure likely to be formed, would be dominated by a bainitic ferrite phase and so would exhibit hardness and tensile properties in excess of the parent material. It is predicted that if the same abrupt transition between unstirred and stirred material observed in aluminium occurred in steel, this would lead to crack initiation, followed by rapid propagation through the relatively brittle weld microstructure. Hence, these findings demonstrate that without further design and process improvements, stationary shoulder friction stir welding is unlikely to be applicable to steel.     

Friction stir welding characteristics of different heat-treated-state 2219 aluminum alloy plates

Friction stir welding (FSW) of 2219-O and 2219-T6 aluminum alloys was performed to investigate the effects of the base material conditions on the FSW characteristics. The experimental results indicated that the base material condition has a significant effect on weld morphologies, weld defects, and mechanical properties of joints. In the 2219-O welds, no discernible interface exists between the stir zone (SZ) and the thermal-mechanically affected zone (TMAZ), and weld defects are liable to form in the lower part of the weld. In the 2219-T6 welds, there is visible interface between the SZ and the TMAZ, and a weld nugget with an “onion ring”-like morphology clearly exists. The defects are liable to form in the upper part of the weld. The strength efficiency of 2219-O joints is 100%, while that of 2219-T6 joints is only up to 82%. In addition, the two types of joints have different fracture location characteristics.     

Effect of friction stir welding process parameters on the tensile strength of dissimilar aluminum alloy AA2024-T3 and AA7075-T6 joints

This work investigates the influence of friction stir welding parameters on the mechanical properties of the dissimilar joint between AA2024-T3 and AA7075-T6. Experiments are conducted consistent with the three-level face-centered composite design. Response surface methodology is used to develop the regression model for predicting the tensile strength of the joints. The analysis of variance technique is used to access the adequacy of the developed model. The model is used to study the effect of key operating process parameters namely, tool rotation speed, welding speed and shoulder diameter on the tensile strength of the joints. The results indicate that friction stir welding of aluminum alloys at a tool rotation speed of 1050 min⁻¹, welding speed of 40 mm/min and a shoulder diameter of 17.5 mm would produce defect less joint with high tensile strength.     

Interface strengthening in dissimilar double-sided friction stir spot welding of AZ31/ZK60 magnesium alloys by adjustable probes

Dissimilar welding of AZ31/ZK60 magnesium alloys with a thickness of 2 mm was successfully carried out by the double-sided friction stir spot welding with adjustable probes.A dissimilar joint bearing flat surfaces on both sides without a keyhole was obtained and the shear failure load of 8.7±0.5 kN was reached.The role of the adjustable probes has been revealed in detail.In the center of the stir zone,the welding interface structure was heterogeneous around which some distinct oxides still remained,leading to a weak interface strength.On the contrary,the welding interface structure around the shoul-der/probe interface was homogeneous with no oxides giving rise to a strong interface strength,which is attributed to the severe material flow introduced by the adjustable probes.In addition,the vicinity outside the shoulder/probe interface,where the fracture occurred during the shear tensile tests,was also strengthened owing to the shearing and torsion by the adjustable probes.Therefore,a stable plug failure can be obtained and the joint properties can be improved.     

Friction stir weld assisted diffusion bonding of 5754 aluminum alloy to coated high strength steels

In the present paper friction stir-induced diffusion bonding is used for joining sheets of 5754 aluminum alloy to coated high strength steels (DP600 and 22MnB5) by promoting diffusion bonding in an overlap configuration. Mechanical performance and microstructures of joints were analyzed by overlap shear testing, metallography, and X-ray diffraction. Our results show that the strength of joint is dependent upon tool travel speed and the depth of the tool pin relative to the steel surface. The thickness and types of intermetallic compounds formed at the interface play a significant role in achieving a joint with optimum performance. That is, the formation of high aluminum composition intermetallic compounds (i.e. Al₅Fe₂) at the interface of the friction stir lap joint appeared to have a more negative effect on joint strength compared to the presence of high iron composition intermetallic phases (i.e. FeAl). This is in agreement with previously reported findings that FeAl intermetallic can improve the fracture toughness and interface strength in Al/St joints.     

The optimisation of process parameters for friction stir spot-welded AA3003-H12 aluminium alloy using a Taguchi orthogonal array

The aim of the present work is to optimise the welding parameters for friction stir spot welded non-heat-treatable AA3003-H12 aluminium alloy sheets using a Taguchi orthogonal array. The welding parameters, such as the tool rotational speed, tool plunge depth and dwell time, were determined according to the Taguchi orthogonal table L9 using a randomised approach. The optimum welding parameters for the peak tensile shear load of the joints were predicted, and the individual importance of each parameter on the tensile shear load of the friction stir spot weld was evaluated by examining the signal-to-noise ratio and analysis of variance (ANOVA) results. The optimum levels of the plunge depth, dwell time and tool rotational speed were found to be 4.8 mm, 2 s and 1500 rpm, respectively. The ANOVA results indicated that the tool plunge depth has the higher statistical effect with 69.26% on the tensile shear load, followed by the dwell time and rotational speed. The tensile shear load of the friction stir spot welding (FSSW) joints increased with increasing plunge depth. Additionally, examination of the weld cross-sections, microhardness tests and fracture characterisation of the selected friction spot welded joints were conducted to understand the better performance of the joints. All the fractures of the joints during tensile testing occurred at stir zone (SZ), where the bonded section was minimum. The tensile shear load and tensile deformation of the FSSW joints increased linearly with increasing the bonded size. The finer grain size in the SZ led to the higher hardness, which resulted in higher fracture strength. When the tensile shear load of the joints increased approximately 3-fold, the failure energy absorption of the joints increased approximately 15-fold.     

铝合金搅拌摩擦焊超声检测研究进展EI北大核心CSCD

铝合金搅拌摩擦焊(friction stir welding, FSW)焊接参数选择不当将会产生隧道孔、未焊透(lack of penetration, LOP)和吻接等取向复杂、细微紧贴的缺陷。首先,本文简述了FSW焊缝与典型缺陷特征,总结了超声检测时面临纵向分辨力低、缺陷表征不完整、材料与缺陷声阻抗接近和灵敏度不足等难点。随后,从常规超声、超声衍射时差法(time-of-flight diffraction, TOFD)、相控阵超声检测技术和其他超声检测技术等方面综述了现有的铝合金FSW超声检测研究工作。最后,结合超声信号处理方法和机器学习方法对研究前景进行展望:可以通过分析和提取信号特征,进一步提升超声检测分辨力和信噪比,并实现取向复杂缺陷和细微紧贴缺陷的精准辨识与定量。     

Microstructural evolution and mechanical properties of dissimilar Al–Cu joints produced by friction stir welding

5A02 aluminum alloy and pure copper were joined by friction stir welding (FSW). A defect-free joint was obtained when one of process parameters, i.e. the traverse speed was lowered from 40 mm/min to 20 mm/min. A good mixing of Al and Cu was observed in the weld nugget zone (WNZ). A large amount of fine Cu particles were dispersed in the upper part of the WNZ producing a composite-like structure. In the lower part, nano-scaled intercalations were observed and identified by transmission electron microscopy (TEM). These layered structures were subsequently confirmed as Al₄Cu₉ (γ), Al₂Cu₃ (ε), Al₂Cu (θ), respectively. Formation of these microstructures caused an inhomogeneous hardness profile. Particularly, a distinct rise in hardness was noticed at the Al/Cu interface. Excellent metallurgical bonding between Al and Cu gave rise to good behaviors in the tensile and bending strength.     

Numerical simulation and experimental investigation on friction stir welding of 6061-T6 aluminum alloy

A modified three-dimensional model was established to simulate the friction stir welding of the 6061-T6 aluminum alloy. A detailed calculating method of the heat generation was proposed by taking account of the contact conditions between the tool and the work-piece. The results show that the heat mainly generated within the region close to the shoulder, the high temperature exists within the upper portion of the weld and decreases along the thickness direction. The strong material flow mainly occurs within the region around the tool and the material ahead of the tool sweeps toward the RS and finally deposits behind the tool. During this procedure the material is extruded to experience different shear orientations, and a defect-prone region exists in the region where material flow is weak. The temperature field and material flow behaviors predicted by the simulation method are in good agreement with the results obtained by the experiments.     

铝/镀锌钢搅拌摩擦铆焊接头组织与力学性能

为实现铝钢之间的优质连接,采用搅拌摩擦铆焊新方法对6061铝合金和DP600镀锌钢进行搭接点焊,利用扫描电子显微镜、能谱仪及拉伸试验对接头的微观组织及力学性能进行了研究.结果表明:接头成形平整美观,中心没有匙孔;接头包含铆接区和扩散区,其中在铆接区铝合金以铝柱的形式嵌入到钢板的圆孔中,形成了一个"铝铆钉",底部有富铝的α固溶体偏聚,圆孔四周形成扩散区,铝和钢形成了冶金结合,依靠金属间化合物Fe Al3连接在一起;接头有3种断裂形式,在最佳工艺参数下接头的抗剪力达到8.2 k N;铝柱上断口的微观形貌是被拉长的韧窝,扩散区的断口由灰色基体和白色颗粒组成.     

Relationship between base metal properties and friction stir welding process parameters

Friction stir welding (FSW) is a solid state welding process for joining aluminum alloys and has been employed in aerospace, rail, automotive and marine industries for joining aluminium, magnesium, zinc and copper alloys. In FSW, the base metal properties such as yield strength, ductility and hardness control the plastic flow of the material under the action of rotating non-consumable tool. The FSW process parameters such as tool rotational speed, welding speed, axial force, etc. play a major role in deciding the weld quality. In this investigation, an attempt has been made to establish relationship between the base material properties and FSW process parameters. FSW joints have been made using five different grades of aluminium alloys (AA1050, AA6061, AA2024, AA7039 and AA7075) using different combinations of process parameters. Macrostructural analysis has been done to check the weld quality (defective or defect free). Empirical relationships have been established between base metal properties and tool rotational speed and welding speed, respectively. The developed empirical relationships can be effectively used to predict the FSW process parameters to fabricate defect free welds.     

Friction stir welding of high-strength aerospace aluminum alloy and application in rocket tank manufacturing

Friction stir welding (FSW) has been widely adopted in aerospace industry for fabricating high-strength aluminum alloy structures, such as large volume fuel tanks, due to its exceptional advantages including low distortion, less defects and high mechanical properties of the joint. This article systematically reviews the key technical issues in producing large capacity aluminum alloy fuel tanks by using FSW, including tool design, FSW process optimization, nondestructive testing (NDT) techniques and defect repairing techniques, etc. To fulfill the requirements of Chinese aerospace industry, constant-force FSW, retractable tool FSW, lock joint FSW, on-line NDT and solid-state equal-strength FSW techniques, as well as a complete set of aerospace aluminum FSW equipment, have been successfully developed. All these techniques have been engineered and validated in rocket tanks, which enormously improved the fabrication ability of Chinese aerospace industry.